Sharing ASP.NET State Service sessions between IIS7.5 and 8.0 doesn’t work

I've got a web farm consisting of 5 Server 2008R2 machines running IIS 7.5. All of them share session state using a single, separate, server running the ASP.NET State Service (this one also runs 2008 R2). Everything is working wonderfully.

However, I've now added a sixth server, this one running 2012/IIS 8.0. For the life of me, I can't get this server to share session state with the other servers.

The machineKey configuration is the exact same (anonymized):

<machineKey validationKey="XYZ" decryptionKey="ZYX" validation="SHA1" />

The application ID is the exact same across all servers (we run multiple websites, but all have the ID created programmatically, config is identical, verified at runtime as well):

/LM/W3SVC/10351/ROOT

All sites also have the same AppPath, also verified at runtime:

D:\SomeSite\SomApp\

The session configuration is the exact same as well:

<sessionState mode="StateServer" partitionResolverType="MyStateServerPartitionResolver" stateNetworkTimeout="30" timeout="20" />

The MyStateServerPartitionResolver class is very simply, just abstracting the fact that we load the connection string through our own configuration (connectionString value is identical on all servers, also verified):

public class MyStateServerPartitionResolver : IPartitionResolver
{
    private string connectionString;

    public void Initialize()
    {
        connectionString = ConfigSettings.StateServerConnectionString;
    }

    public string ResolvePartition(object key)
    {
        return connectionString;
    }
}

All sites run on .NET 4.0 and the application pool configuration is the exact same as well.

The only difference I can narrow it down to is the fact that the working servers are all running IIS 7.5 while the non-working one is running IIS 8.0. However, I'd expect this to be a supported scenario as it's hard to do a rolling upgrade otherwise.

Any suggestions on what I can do to debug this? Or any documentation confirmation that IIS 7.5 & 8.0 can't share session state?